Why is the CO2 level lower in my house than outside?

[OmCheeto]

The more I study this problem, the harder it gets.
I was thinking that maybe a refrigeration type dehydrator would remove the 'relative humidity' factor from the equation, but then I remembered that water absorbs CO2, and such a device may be scrubbing CO2 from the sample.

Low and behold, the same problem seems to exist for silica drying agents:

Errors in measurements of CO2 with the use of drying agents

"...silica gel (145 g anhydrous weight, mesh 6-20) adsorbed approximately 2 mmol CO2 (38% of total) over a period of 15 min..."​

 

[Woody I]

Your carbon dioxide readings indicate you may have exhaled on or near the sensor. Keep the sensor several feet from your nose, six feet is a good separation. Wait awhile before recording the readings in order to give the sensor time to sense the atmosphere of interest rather than the atmosphere entrained around the sensor from another location or your breath. Perhaps the instrument manual will indicate the time to respond to a change in concentration. 90 seconds is response time for a TSI Q-Trak. If the manual does not indicate the response time, you can exhale on the sensor and note how long it takes to come down from the peak concentration to the ambient concentration.